High rigidity and lightweight are required for a case of a structure such as a transmission which is mounted on a vehicle. Therefore, the case is often formed by using a metal material such as aluminum entirely. A rib may be formed on the metal case for a structure. In this case, a designer may use alloy having small specific gravity as the metal material of the case to provide the case with high rigidity. In addition, the designer may also set a thickness of the case to a small value. However, the designer may face limitation of weight reduction even when the rib is used since the metal material is used for the whole case.
Resin may be used for a part of a case of a structure. The case having a structure in which a metal frame member and a resin cover member are combined is well-known. Patent Document 1 discloses a side cover which forms a part of a case of a transmission. The side cover disclosed in Patent Document 1 includes metal frame members and a resin cover member. The cover member covers an opening portion formed between the frame members.
As taught in Patent Document 1, the combination of the metal frame members with the resin cover member contributes to high rigidity and weight reduction of the case of a structure. However, further weight reduction is required for the case of a structure in order to improve fuel economy of a vehicle.